Btrfs-progs: make btrfsck fix backrefs that are broken

Btrfs-progs: make btrfsck fix backrefs that are broken

If you set an file extent item's disk_bytenr to something completely wrong we
won't be able to fix this if it is the only one who has a ref on the original
disk bytenr.  Our extent records know exactly who is supposed to point at them,
so if we have an extent record that has no backrefs we can go and try to lookup
the backrefs ourselves.  If these backrefs do not point to an extent record that
was actually found then we can be pretty sure this extent record is valid and
the backref is bogus.  Then the verify_backref code can do its thing and reset
the backref to point to the right extent record and we can all carry on.  This
fixes a user reported corruption.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>